1. Field of the Invention
The present invention relates to a metal sinter production method for forming and sintering metal powders.
2. Description of the Prior Art
The powder metallurgical industry has expanded into the sintering field of titanium alloys and hard metals from the conventional irons, low-alloy steels and high-alloy steels.
While the powder metallurgy has developed aiming at the production of components which are difficult to make from ingot materials or at the elimination of cutting operations, in the field if melting and casting any improvement in the performance of metal materials has required an increase in the amount of the solute constituent with the inevitable deterioration of the performance due to segregation of the solute constituent and thus the powder metallurgical techniques have been noted as means of solving the problem.
Under present circumstances, however, the powder metallurgical industry has not grown as expected.
Some causes are conceivable for this situation and for one thing the properties of a raw powder must be considered to constitute a cause. In other words, the performance of a powder metallurgical product is largely dependent on the properties of the raw powder. Of the properties of the raw powder, the oxygen content is the most serious problem. Oxygen is usually present in the form of an oxidation coating on the surface of the metal particles and it impedes the sintering of the metal particles. Thus, the resulting sinter fails to exhibit satisfactory mechanical properties.
It has been known to add chromium, manganese, silicon, aluminum, titanium, vanadium and the like to improve the mechanical properties of a sinter. Many of these addition elements are high in affinity for oxygen so that the raw powder tends to be oxidized easily during its production and handling and, once oxidized, its reduction is difficult. Thus, the alloying elements remaining in the form of oxides rather deteriorate the mechanical properties of the sinter.
For instance, Mn-Cr type low-alloy steels, the most widely used low-alloy steels for mechanical structural purposes, usually contain oxygen in the range between 1500 and 5000 ppm and therefore their quenching properties are deteriorated considerably by their oxygen contents.
While the silicon content is kept low in any of these low-alloy steels, this is due to the difficulty to reduce the silicon carbides in the raw powder by the ordinary solid reduction and as a result the application of silicon to the powder metallurgy is limited considerably.
For instance, a stainless steel powder contains reactive elements such as chronium and silicon and the inexpensive water spray process is used for its production thus frequently resulting in the oxygen content of 1000 to 2500 ppm. However, the reducing treatment of the powder is not easy and the powder is frequently used in its form just resulting from the water spraying. Thus, the resulting oxides, such as, SiO.sub.2 formed on the surface of the particles cause the occurrence of necking during the initial period of the sintering and they also retard the subsequent material transfer. Also, such oxides remain in the form of pseudo grain boundaries and inclusions in the sinter and they also deteriorate such properties as tensile strength, ductility and toughness.
For instance, a nickel-base hard metal powder contains reactive metals such as chromium, titanium and aluminum in large amounts so that a non-contaminating spray-process, e.g., an argon gas spraying is used and the oxygen content is reduced to less than one tenth of that obtained by the water spraying. However, the occurrence of pseudo grain boundaries by the oxidation of the surface of the particles and the like is still a problem as a cause of defects in the sinter.
This problem is more serious in the case of titanium alloys. The reason is that the principal constituents consist of reactive elements as in the case of a Ti-6Al-4V alloy.
As described hereinabove, the state of things is such that in the manufacture of sinters of metals such as irons, low-alloy steels, stainless steels, hard metals and titanium alloys as well as high-speed steels, tool steels, mar-aging steels and magnetic alloys, the surface oxidation of a raw powder causes the occurrence of sintering defects thus failing to fully derive the mechanical properties of the material which should primarily be obtained. It is also known that the surface oxidation of raw powders results in deteriorated magnetic properties in the case of magnetic alloys.